Basic Home-Built CuCl/CuBr Laser Information

The copper chloride (CuCl) and copper bromide (CuBr) lasers are variations of
the copper vapor laser. The Cu vapor laser runs at a high temperature of
around 1,100 °C and is driven with a high voltage pulse, usually
at a repetition rate of 8 to 10 kHz. Its output is roughly split 50:50
between the green (510 nm) and yellow (578 nm) wavelengths.

By starting with a halide of copper rather than pure metal, the CuCl and CuBr
lasers can operate at lower temperatures than required to produce
copper in vapor form. However, the copper must be dissociated from its
halogen atoms to lase. For this reason, power requirements are somewhat
unusual in that a pair of high voltage pulses in rapid succession is needed to
operate the laser: The first separates the Cu and Cl or Br atoms and the
second pumps the Cu atoms to the required upper energy state for lasing to
take place.

Since, the lifetime of the separate atoms is short, this must be repeated for
each activation of the laser. This makes the power supply design a bit more
interesting than the run-of-the-mill neon sign transformer! Also they are
pulsed lasers but at a high enough repetition rate, the output will appear
continuous.

This can be accomplished by a simple but brute-force motor driven distributor
somewhat like that used in an automotive ignition or by a fancy sophisticated
solid state power supply. The former looks and sounds like something out of a
bad Sci-Fi movie (or nightmare, take your pick) but works!

A quasi-CW CuCl/CuBr laser can also be built (and this approach may be used
commercially). At a repetition rate of several kHz, enough copper ions
exist in the dissociated state so that lasing occurs on every pulse. In
other words, each pulse does double duty. Building
a suitable pulser is, however, left for the advanced course. :)

Output wavelengths are green (510.6 nm) and yellow (578.2 nm) and the pulses
can be quite intense!

Like the N2 laser, CuCl and CuBr lasers do not need a resonator to operate.
The design in "Light and its Uses" uses a mirror at one end and an
*unsilvered* piece of glass at the other (output) end. (Of course, plain
glass will act as a partial reflector - reflecting about 4% per surface
at 90-degree incidence, orthogonal to the surface.) So, if the surfaces
are reasonably parallel to each other, an ordinary piece of glass will
reflect between 7% and 8% of light incident on the piece at 90 degrees.
Mirrors can be internal or external (the Sci-Am design used
internal mirrors). Where they are external, optical windows at a
slight angle seal the ends of the tube. The angle is just to minimize
reflections that could affect the resultant mode pattern - it doesn't need
to be (and probably shouldn't be) at the Brewster angle.

(From: Steve Quest (Squest@galileo.cris.com).)

Most lasers need to resonate to build up emission. However, copper vapor
lasers would damage themselves if they were allowed to resonate. There is so
much photonic emission you only need one mirror, the back mirror in the cavity
of a Cu laser. One pass through the cavity is enough, Cu lasers produce
hundreds of watts per pulse!

There are four areas of safety considerations for the home-built CuCl/Cubr
laser (and other similar metal halide or metal vapor lasers, for that matter):

Laser output: The home-built HeHg laser may be capable of producing
a beam with an average power of several 10s of mW at multiple wavelengths.
Since there doesn't seem to be any hard information on the actual power
obtainable with this or similar home-built laser designs, it is important to
take precautions assuming a higher power until determined otherwise.

Avoid eye contact with the direct or reflected beam. This includes the 4
beams reflecting off the Brewster windows which may be quite strong.

Electrical: The power supplies can be lethal. Neon sign transformer
based power supplies have enough voltage and current to stop a heart, Even
if you aren't killed, the shock may startle you into doing something you might
regret. Make sure you read and follow the Safety
Guidelines for High Voltage and/or Line Powered Equipment. Insulate all
connections and install barriers to prevent contact with the high voltage.
And, don't forget about the terminals of the heater!

High temperatures: The laser tube operates at several hundred
°C. Burn hazards may be present for portions of the tube that extend
beyond the fire brick enclosure as well as parts of the bricks themselves.

Rotating machinary If you use the rotary pulser, in addition to its
electrical dangers is the risk of getting caught on rotating parts and being
ripped to shreds or sucked into the high voltage! In addition, there is a
very real danger of an inadequately rated disk flying apart and causing
serious injury.

(From: Rich (synergyvideo@hotmail.com).)

"My impatience made me fire up my pulser to measure the rotation rate
of the disk. Imagine my surprise to find that the plastic I
used couldn't take the stress of 6,000+ RPM! After I got back from
the hospital the other night with shiny new staples in both arms I
found pieces of plastic stuck in 3 walls and 4 different rooms.
Thankfully I at least had safety goggles on....
Needless to say, I'm a bit gun-shy at the moment with rapidly spinning
disks, so I'm pursuing the solid-state power supply instead."

Provide proper warning signs for both the laser radiation and high voltage.
Surround the mechanical pulser (if used) with a sturdy shield.
Keep pets and small children out of the area and make sure everyone present is
instructed as to the dangers. The use of proper laser safety goggles for the
specific wavelength(s) of your laser are highly recommended.

Information Unlimited (IU),
MWK Laser Products, and
Bull Electrical
(UK), sell plans for a copper vapor laser. The IU plans are apparently
based on the paper summarized in the next section.) The others may also be
based on this work or the article from Scientific American but I don't know
for sure.

The first paper below appears to describe the copper vapor laser upon which the
plans from Information Unlimited (and
possibly others) are based.

(From: Steve Roberts.)

Gabay, Smilanski, Levin, and Erez, "Comparison of CuCl, CuBr and CuI as
Lasants for Copper Vapor Lasers", IEEE Journal of Quantum Electronics, pg.
364, May 1977. This is the Negev Nuke Research Copper Vapor Laser (CVL).

It used a 25 mm quartz tube tube in a standard 30 mm lab tube furnace with
flowing helium at 4 to 8 Torr. Although we now know that neon is superior for
this task, helium is a heck of a lot cheaper. Mirrors are flat with a 100% HR
and a 60% transmission OC. The discharge circuit doesn't have a peaking cap
across the tube. That came later from other researchers and really boosts
power. The halide is in a dimple in the middle of the tube.

Optimal conditions were: CuBr at 420 °C (measured at tube wall),
8 Torr of helium, 260 us delay between pulses at 5 Hz. See
NNR Double Pulse Driver for Copper Vapor Laser.
(This was redrawn based on Steve's scribbles from the paper. --- Sam)
The left side thyratron in the schematic provides the dissociation pulse,
the right side thyratron with smaller cap provides the lasing pulse. No
attempt was made to optimize the circuit for a faster rise time according
to the text. 5 grams of CuBr lasted for around 20 hours of operation.

They used aluminum blocks as electrodes and Teflon blocks and O-rings to
hold the works together and hold the Brewster stems as well, thus having
long inactive regions. The Brewster stems were separate Pyrex pieces with
quartz Brewster windows attached with Epoxy.

A better tube design was later published by the same group (IEEE Journal of
Quantum Electronics, Nov. 1978).

This used a 2.5 cm diameter quartz tube in a standard lab oven, 30 cm long
active region. The rest of the tube was constructed of Pyrex using standard
quartz-to-Pyrex transition seals (usually rings of 6 different glasses in
series) with Pyrex stems and tubular aluminum electrodes. Like the first
desgin the cavity used planar mirrors and they were 1 meter apart. Both
lines lased with 1.5 mR divergence, the beam diameter being limited to 1.2
cm by the Brewster windows. It would lase superradiantly without the mirrors.
The rep rate was 30 Hz using an EG&G 1802 thyratron and a 0.2 uF cap on the
dissociation side and a triggered spark gap and .012 uF cap on the lasing
side. No details of the spark gap are given. Both sides charge through 1M
ohm resistors and the 4 uH inductor is absent. Minimum voltage for the
disassociation pulse is 7 KV and lasing is a 24 ns pulse, lengthened by gain
passes between the mirrors. The optimal firing delay is 200 us. This laser
operated at a temperature 550 °C using CuI.

Also see the section: Chris's Copper Halide Laser for a description of a
copper halide laser very similar to the one described in "Light and its Uses".

Although the complexity of this laser may appear at first to be greater than
that of some of the others, the required skills at each step are often more
modest so overall, the likelihood of success may be greater!

Mirrors - Only one silvered (aluminized) mirror needed. Output coupler
is a high quality microscope slide with approximately 4% reflection.
Adjustable mirror mounts are attached directly to aluminum laser tube
extensions. Alignment by sighting down the bore from a meter or so distance
looking for eyeball reflection with mirror adjustment performed by an
assistant.

The HV rectifiers are each made from a string of 50 (!!) 1N4007s (1000 V,
1A) diodes in series. Of course, more sane alternatives are possible
such as 4 microwave oven HV diodes in series. (See the section:
Standard and Custom HV Rectifiers.)

C1 and C2 are the energy storage capacitors of about .015 uF at 25 kV.
They are made from alternating layers of aluminum foil and plastic sheets
with a total area of about 1,500 square cm. The thickness of the plastic
sheets is not indicated. (See the section:
Standard and Custom HV Capacitors.) They
are mounted close to the discharge tube to minimize problems with stray
inductance due to lead length.

C3 and C4 are optional smoothing capacitors which would allow power to be
drawn through the pulser regardless of its phase with respect to the AC
line. They are of similar construction to C1 and C2.

R1 and R2 are made from a network of 1,500 ohm, 2W, in a series parallel
combination. It isn't quite clear what this means! However, they DO need
to be rated for both high voltage (20kV) and high power. See the section:
Power Supply Construction Considerations
for some comments on high voltage resistors.

R3 and R4 are safety bleeder resistors. The capacitance (C1+C2 and C3+C4)
is several times that of a typical CRT - and you don't want to be messing
with one of those when it is charged! A value of 100M ohms, 5 W (rated for
20kV operation) would result in a time constant of 3 seconds or less but
this still means that to discharge to a safe level could still take 20 or 30
seconds.

There doesn't appear to be any reason that a single 15 kV, 60 mA NST
can't be used with a bridge rectifier to feed C3 and C4
instead of two separate units - half their output is not being used.

Also, even a single lower current NST such as
15 kV at 30 mA should also be satisfactory but the pulsar would need
to run at a lower repetition rate. A lower voltage may even be acceptable
(e.g., 12 kV) but construct the circuit assuming the higher voltage
to simplify upgrading later.

These set of guidelines should be followed during construction of your first
home-built CuCl/CuBr laser. The factors below will greatly influence the
ultimate output power, beam quality, and whether it produces any coherent
light at all! Once you have a working laser, feel free to make modifications
- one at a time.

The main recommendation for your first CuCl/CuBr laser is to NOT make
significant changes to any of the basic specifications (dimensions, materials)
of the laser assembly and power supply.

The motor driven dual pulse power supply is simple, if clunky, but does
work. If you want to build a fancy solid state pulser later, go for it.
But, do the crude noisy one first!

Other Examples of Home-Built CuCl/CuBr Lasers

My copper halide laser is similar to the original article in "Light and its
Uses" but with some updated components used in construction. For instance;
instead of a quartz heater next to the plasma tube, I employed high
temperature, laboratory type, heating tape wrapped directly around the plasma
tube. This was purchased from AmpTek
Company. I used the medium watt
density, 1/2 inch wide tape. The 36 inch length is the perfect length for
wrapping the 24 inch, 14 mm OD quartz plasma tube, covering 21 inches using
closely spaced wraps. This product is capable of reaching 1400 F. The
temperature of the laser is controlled by a process controller with a digital
readout of the actual temperature via a thermistor mounted very close to the
plasma tube. This unit was calibrated using a temperature probe inside the
plasma tube before the laser was completed.

The plasma tube is constructed of heavy walled quartz tubing to avoid the
possibility of the high voltage plasma arcing to the heating tape. The 2 mm
fused quartz walls have a breakdown voltage of between 30,000 and 40,000
volts. Well above the 15,000 volts passing through the plasma tube. The
Diagram of CuBr Laser shows the general construction details.

The high voltage power supply uses a mechanical distributor to double-pulse
the laser at the required repetition rate. The motor (salvaged from a clothes
dryer, I believe) is rated at 1725 rpm (but probably actually running close to
1800 rpm under these essentially no-load conditions). With the size if the
sheaves I am using this has the pulser electrodes spinning at approximately
7000 rpm. This is something that can and should be adjustable anyway. The
speed of the electrode assembly can affect this lasers operation. Just
another parameter to play with to optimize this laser's output.

WARNING: The potential danger from the energy stored in C1 and C2 cannot be
overemphasized! R1 and R2 are 100M ohm bleeder resistors rated at 5 W and
20 kV, which safely discharge these capacitors in about 10 seconds once
system is shut down.

Two pulses separated by 150 microseconds are needed to disassociate the
copper from the halide and then excite the free copper atoms. This is repeated
approximately 115 to 120 times per second (determined by the distributor motor
speed and mechanical linkage). Low induction connections are needed between
capacitors, pulser and plasma tube electrodes.

I am building a CuBr laser using a power supply based on a Marx generator.
I use three, 2 kW microwave oven transformers connected to three-phase AC
source and a three-phase (6 diode) bridge rectifier and 10 uF filter capacitor.
The charging caps are made from 25 individual 2 nF, 7.5 kV ceramic capacitors
soldered on two copper stripes (to keep inductance low and impulse current
high).

The copper chloride (CuCl) laser produces powerful bursts of green (510.6 nm)
and yellow (578.2 nm) light. Pulsed at 50 hertz the output appears continuous.
I have been searching U.S. patents and have found a wealth of information on
various types of metal vapor lasers. Great stuff!

I have just completed a home built copper halide laser. I decided to use
copper(II) bromide (in helium) instead of copper chloride as the lasing
medium. I had read that this copper halide was more efficient than the
chloride. Also of interest is the addition of a small amount of hydrogen to
the mix to dramatically increase output power. I have yet to try this, as I
just obtained the copper(II) bromide. I was very fortunate to produce a
beautiful, bright, green beam on the very first try. This is somewhat unusual
with home built systems.

I am currently using a mechanical pulser (similar to the one described in the
Scientific American article) to produce the closely spaced, rapid, high
voltage discharges required for this laser's operation. This leaves something
to be desired, as it is very noisy. I had considered the use of thyratrons for
this application until I saw the price of these. (YIKES!!)

My copper bromide laser is very unique in many ways. It is certainly the
first laser I have ever built that not only requires eye protection, but ear
protection also. :-) The pulser, when in operation, is very LOUD.

It is very difficult to determine the power output of this laser without an
expensive laser power meter. I unfortunately, cannot afford a power meter
capable of reading such powers as this laser may produce.

I have been doing some more "playing" with this halide laser - experimenting
to determine the effect of pressure on its operation. The laser seems to
operate best below a couple of Torr. I was getting extremely bright pulses of
both green and occasionally bright yellow, with a marked improvement in
repetition rate. The beam diameter has also become much larger than it was at
higher pressures. It still does not appear continuous but this may be able to
be remedied by adjustments to the pulser.

All was going fine for an hour or so until one of my capacitors failed. I am
using copper clad PCB caps and these are prone to dielectric breakdown every
so often. There was a small hole blown through right at one corner. The
potential at these sharp corners is more concentrated and this is usually
where they fail. This makes for an easy fix, though. I have the damaged corner
soaking in ferric chloride as we speak. It should be good as new very soon. I
will be more careful in limiting voltage to these from now on. I guess I got a
little carried away watching this beautiful beam become more stable at higher
charging rates.

Chris Krah and I have been conversing
almost daily as to this laser project.
As a matter of fact, I don't think I would have built this laser if it wasn't
for his interest.

The thought of a solid state power supply is really what 'sparked' my
interest. I was hoping I would not have to build a mechanical pulser for this
project. Once the laser head was completed though, I just had to see it
operational and couldn't wait for Chris to finalize his PSU.

In a universe far, far away, during a time in the deep dark past before Chris
Krah built his CuCl laser.....

(Questions from: Chris Krah (chriskrah@apple.com))

"Can somebody describe briefly how a copper chloride laser works? How
hard/costly is it to construct such a laser? Is it appropriate for materials
processing?"

(Replies from: Steve Quest (squest@cris.com).)

Briefly describe how a copper vapor laser works? :) Ok, I'll try.

First, you have a non-resonant cavity, just a Brewster window on one
end and a full silver on the other. Heaters heat the cavity (vacuum)
until the cupric chloride vaporizes inside. (Note: This differs from the
recommendation in SciAm and elsewhere to put a low reflecting mirror like
a microscope slide at the OC-end. --- Sam) A high current, high voltage
discharge is fired through the cavity to dissociate the Cu from the Cl
resulting in free copper vapor without the need to heat copper to the
boiling point. At this time (very quickly after the dissociation pulse)
another pulse is fired to excite (pump) the copper atoms. Spontaneous
emissions that hit the mirror and fly back through the cavity stimulate
the laser emission which exits the Brewster window. Such lasers are
incredibly powerful, and the wavelength is visible in the yellow band.

The answer should come to you if you do a ray-trace. Plano is the only mirror
choice that makes sense, as concave would redirect much of the photons into
the wall of the cavity.

"How "good" does the vacuum have to be? (Please tell me it's not a total
vacuum such as in vacuum tubes). Do you need a vacuum at all? (It would
certainly reduce the efficiency)."

You need to get the air out, all nitrogen and especially oxygen. A good
vacuum produced with normal vacuum pumps (like when you pump down an argon
tube before re-gassing) is good enough.

"What kind of heaters are used? I would think that an induction heater would
do."

Quartz halogen heating elements, a quartz tube for the cavity, and insulate
the thing well. It takes a lot of heat 600 °C if I remember correctly,
to bring cupric chloride to the vapor state. HOWEVER, using this method
prevents you from needing to bring copper to the vapor state, an act that
would be impossible inside a quartz cavity (the quartz would melt and suck in
way before the copper vaporized.

"What if you turn of the laser off ? Do you have to evacuate the Cl/Cu or does
it turn to copper chloride when you cool down the cavity?"

Once the laser fires (single pulse), the cu and cl recombine, and the next
dual-discharge dissociates and excites for the next pulse. This rep rate can
run as high as 300 Hz, but slower is better, if I remember correctly. When
you shut down the laser, the cupric chloride redeposits as a solid on the
glass of the cavity (on the walls) and waits until you heat it back up again.
My prototype was purely experimental, and the output power amazed even myself!
And I'm hard to impress anymore. :) However it's been a few years so much of
my memory is fog these days. ;)

"Seems awfully hard to construct."

I thought it would be, but it wasn't so bad. The gold beams are intense,
quite a sight to behold! If you're into beam shows, this is your laser of
choice, much better than an arc-lamp pumped Nd:YAG with a KTP doubler. :) Uses
about half the electricity to produce twice the beam!

(From: Jon Singer.)

This disagrees with the literature. The maximum rep rate
for copper vapor is now above 200,000 Hz, and I'm fairly
sure that best output was obtained at something
*far* in excess of 300 Hz. Here is a reference for CuBr:
"A copper bromide vapour laser with a high pulse repetition rate",
D. V. Shiyanov, et. al. 2002 Quantum Electron. 32, 680-682.

(From: Chris Krah (chriskrah@apple.com).)

Thanks for your quick response. :)

So in summary a copper vapor laser would consist of the following components:

A quartz cavity between Brewster window and plano mirror.

The quartz cavity would be equipped with:

A vacuum nozzle.

A pair of electrodes (one at each end of the cavity).

Halogen heating elements.

Power supply.

I assume that the electrodes for the dissociation pulse/excitation pulse are
the same.

The time gap between dissociation pulse and excitation pulse should be
dependent on how fast the chloride and copper atoms recombine to copper
chloride.

I assume the power output should be dependent on:

length/diameter of cavity (how many copper atoms are stimulated)

excitation voltage (how many copper atoms are stimulated)

dissociation voltage/current (how many copper atoms are available)

temperature / pressure inside cavity (see 1)

I would think that (3) could be calculated by applying Faraday's Law and (2)
by using the information in (3) and E = hf, (3) and (4) by using
thermodynamics laws. Based on this information the power output could be
calculated.

Is there a rule of thumb to estimate the power output?

Power supply:

Since a pulsed output is required a neon transformer probably makes no sense
(unless you use a vacuum spark gap or thyratron -> expensive)

What power supply did you use? I think a SCR triggered ignition coil could be
used (similar to a trigger transformer setup in a flashlamp application ).

Copper vapor is a high gain medium meaning you can make them fairly small,
although, even with limited space, I doubt there would be a hell of a lot
of difference in the overall size of an amateur built laser that was putting
out 10 W, or a few hundred milliwatts.

Here are some general guidelines for high power copper vapor laser
construction:

Optimum bore diameter around 2 cm.

Optimum discharge voltage around 200 V/cm.

Optimum copper heater temperature around 1,400 °C.

Neon buffer gas pressure of 25 to 35 torr, although this is dependent on
length. For very long tubes, optimum buffer pressure may be as high as 100
Torr (helium can also be used on smaller lasers, and the voltage requirement
of the tube is roughly 1/2 that of neon, pressure is roughly 1/4 of quoted
above. However, using helium as a buffer does not allow the tube to be
scaled well, and output power, all other things being equal, will be roughly
2/3rds that of a system using neon.)

An unstable resonator is normally used for a copper vapor laser. However,
short cavity flat-flat works well also. Since copper vapor is high gain,
in very large units a normal piece of glass can be used as an effective OC.
For smaller lasers, a partially reflective mirror may be used.

The above information has been gleaned from LLNL's AVLIS program (the early
years). The largest CVL they made had a 7 cm bore, and was just over
a meter long. it produced nearly 90 W of green as an oscillator, and they
were able to extract 120 W of power out of it in a MOPA (Master Oscillator
Power Amplifier) configuration. With copper vapor's high gain, if you don't
want to scale the laser, you can always build a power amplifier.

(From: Sam.)

I think I have a (reject) mirror from that or a similar LLNL laser, rated
for 1,000 W(!!) and it is quite spectacular: Over 76 mm in diameter
coated for 99.996% reflectance in the range 511 to 578 nm, 1/20 wave surface
finish. Need I say more. :) The only defect is a tiny internal fracture
near one edge which doesn't affect anything optical. I have to be
sitting down to just think about the original cost of that mirror. :)

(From: Steve Roberts.)

It's a hump shaped curve. You can increase it too much. Are you running
pure copper or a halide? Is your discharge self heated? Don't forget that
the buffer gas also has a large impact out output power. The buffer gas
transfers energy to the copper via collisions when it's excited (sort of like
the function of the helium in a HeNe laser). You can't use air or nitrogen,
as they will combine with the copper - it will burn. Acceptable buffer
gases are the inert gases neon, helium, and argon. Neon produces
the greatest output, followed by helium. Argon produces a much lower output
power. Assuming you're in the USA, helium is cheap in welding grade gas,
about $80 to purchase the tank and $15 a refill. (That may have been true
at one time, but it is a lot more expensive now!)

You need to be hot at all times, it depends on your tube construction. You
may need to reduce the heat, but without a professional pulser running at
10 to 20 kHz, you will need the heater. This depends much on your tube
construction and what you use to insulate it. Ceramic wool from a lab
supply catalog can take the heat and would be my choice for insulator.

There are different curves for copper vapor and copper halide lasers. The
book you need is:

It's in English, Broken English translated by a Russian techie type, but
usable. Every paper it references is Russian and thus probably unobtainium
unless you have friends there skilled in spectroscopy. 300 dense pages. I
got it on interlibrary loan, but I'm at a university and that makes it
easy.

For one of their poorer examples:

They have about 1.5 to 2.0 kW average power measured at the rectifier. 300
ns pulses at 200 to 300 A but they use a carefully selected peaking capacitor
across the tube, which is the real key to operation, i.e., a sloped rising
edge. Rise time is 45 ns. That's for 2.1 W in a 12 mm bore and 3.75
W in a 20 mm bore using Ne at 12 Torr in a 60 cm long tube. 10 kHz rep
rate. Temperature is 390 °C using CuBr. It is not however a simple
straight wall tube, but has disks. Output is approximately 3.5 W of green
and yellow. Scaling the bore to 22 mm and raising the rep rate to 15 kHz
resulted in around 7.3 watts from the same PSU. This is a resonator (with
mirrors), not with just windows, in an external oven.

If that design was switched to Chris Little's HYBRID gas mix (CuBr with H2 in
Ne), which was developed after the book was written, it would probably be even
better. Little has the current record for CVL power per watt.

Their list of gases:

Neon - Best by a factor of 2.

Helium - Good.

Argon - Fair.

Krypton - Barely lases.

Xenon - Kills lasing entirely and is unstable in the discharge.

As in all books Russian, at least those before Capitalism hit over there,.
it gives the complete details on everything. PSUs, tube seals, heating, the
works. They get 7 to 20 watts from 1 meter long quartz tubes using copper
bromide and external heaters. The secret is to scale the PRF high enough i.e.,
14 to 20 Khz, that you don't need to do a double pulse. This of course means a
thyratron and a pulse forming network, or in some cases two thyratrons in
push-pull.

You don't need or want a Brewster window on the laser's output-end.
The Brewster's angle is calculated from the window material and the
wavelength of interest. The Cu halide laser will produce two distinct
wavelengths, one at 510.6 nm and the other at 578.2, so the angle could
not be optimum for both. Secondly and more importantly, Brewster windows
are almost exclusively sealed to the ends of a plasma tube. During
operation of a metal vapor laser such as this, deposits will make there
way to the windows and they will have to be cleaned periodically. Using
easily detachable mirror mounts as opposed to sealed windows will make
this task MUCH easier.

But, then we have the sealed tube CVLs:

(From: L. Michael Roberts (NewsMail@LaserFX.com).)

I own a 5 watt Spectronika copper vapor laser and it does not have brewster
windows. It does not even have a front mirror. The front-end has a window on
it (non removable as this is a sealed tube design). The back-end has a window
with a mirror mounted on a metal ring. This is a copper bromide system
using a multi-kHz pulse excitation.

Spectronika sell the replacement tubes for their laser for only $1,650 so it
might almost be worth it to buy the tube and build the supply yourself.

The following technical information is from: CRC Handbook of Chemistry
and Physics, CRC Press, 56th edition.

400 °C is only 752 °F and not 1,400 °F, but the melting
oint of CuCl is 430 °C or 806 °F and the boiling point of CuCl
is 1,490 °C or 2,714 °F. This, of course requires a temperature equal
to or greater than 810 °C or 1490 °F to be the optional point at
which enough CuCl vaporizes under non-standard STP for lasing to take place.
It may be interesting to note that the point at which CuCl boils is obtainable
with a quartz tube, since the exact melting point of quartz is 1,610 °C
or 2,930 °F. For comparison, iron melts at 1,535 °C or 2,795 °F,
or just after the boiling point of CuCl.

Here are some notes about refactories and gas fired approaches.

So just for fun, what would withstand these temperatures? Well, the answer
is that plane old Portland Cement makes an excellent high temperature
refactory when mixed with ground yellow fireplace brick (Denver Fire
Clay) and applyed in thin layers of less than or equal to 1 inch thick
with carved/shaped/worked fire brick as the supporting structure. In other
words, the Portland paste as the heat shield for the Denver Fire Clay, as
the latter will melt down at about 2,500 °F (orange/yellow heat)
but an inch of the Portland-Fireclay mixture will protect it completely
in yellow/white heat. Blast propane and compressed air into the cavity
and you can now completely vaporize the cuprous chloride and maybe run the
laser at slightly higher pressures. Just remember to protrude extra
quartz from the fire box as not to burn the tube seal. How good is
calcium aluminum silicate as a refactory? 3,200 °F, about the
hottest you could ever get with propane and forced air from a straight
piece of steel pipe. The temperature can be controlled by adding too much gas to
too little air or visa versa but too little air is much better as it keeps
oxidation under control, and I have never had any problems with the
refractory paste except if I were to spill other reactive superheated fluids
on the hardened mixture which would cause it to break down requiring spot
patching. I never had it explode, but it will crack if it's too thick. It
might also be interesting to know that 2,500 °F can be reached
with most Nichrome heating elements, but after long runs at peak
temperature the elements will harden. So make the box such that the elements
are supported and expect to replace them every 200 hours. The real trick to getting high
temperatures out of electric elements is using highly reflective
refactory like ceramic fiber "fluff" resembles R11 insulation but more
dense, or use more elements. If you asked me what the best refractory
ever is, I would say the fluff: it can be blinding white hot then warm to
the touch in about 15 seconds. If you use "fluff " for anything keep in
mind that serious unexpected peak temperatures are possible that you
wouldn't have expected because of its excellent ability to reflect heat
yet remain hot on just the surface. It is this property that allows the
entire furnace to reach heat soaking of within 95% of the peak
temperature of the heat source even if the heat source is small. I tend
to use fluff whenever possible and Portland cement makes a good adhesive
for the "fluff" as well

For more fun... It sure would be nice if the boiling point of copper was
not 2,567 °C which is an amazing 4,652.6 °F and is equal to the
filament temperature of a 120 watt light bulb! One other interesting fact
is that most forms of copper chloride decompose at red heat back to the
simple CuCl we have all come to love.

Here is a list of those that decompose when heated to 800 °C:

Copper (II) chloride: CuCl2.

Copper (II) chloride, basic: CuCl2.Cu(OH)2.

Copper (II) chloride, dihydrate: CuCl.2(H20).

And I now part with this question: What is the best sealant that can
withstand temperatures of greater than 400 °C? for anyone that may
read this and knows the answer.

I need to design a high voltage pulse transformer. Input voltage is about 200
V (at several 10s of amps). Output voltage 20 kV. The pulse transformer
needs to step up two closely spaced 50 us wide pulses (pulses are spaced at
200 us).
The problem: The HV pulses at the output of the transformer have to be sharp
edged just like the input pulses. A regular ignition coil won't work. The
pulses appear as one wide pulse at the output. Therefore I guess it is
necessary to build my own pulse transformer. My guess is: Use core material
with low permeability to keep in and output inductance low.

(From: Bill Sloman (sloman@sci.kun.nl).)

Of course, if you want to preserve the sharp edges you need a transmission
line transformer. These can be designed to produce integer step-up ratios -
see: R.E. Matick, Proceeding of the IEEE, volume 56, pages 47-62 (1968).
However, I've never heard of one designed for a 100:1 step-up.

Two 10:1 stages might be practical, and three 3:1 plus a 4:1 get you back to
familiar territory, though with 5kV between the turns, the top transformer
probably has to be wound with good quality RG58CU.

The other advantage of transmission line transformers is that you can use high
permeability material for your core, since this only has to handle the
low-frequency droop.

The disadvantage is that it is an incredible pain to comprehend what is going
on - but Winfield Hill seems to understand them, so it is humanly possible.

For the life of me, I can't work out whether you could get away with toroidal
strip-wound cores. Classical high voltage pulse transformers used soft-iron
wire cores, but I don't know if this is relevant. I once found a nice text on
the subject in the Southampton University library in 1972, which I remember as
"High Voltage Pulse Technology" by Frugnel, but after 25 years this can't be
relied on. There certainly isn't anything with this title in the Dutch
academic library system.

(From: Chris Krah (chriskrah@apple.com).)

A 100:1 step up transmission line transformer would require 10 transmission
lines. Please correct me if I am wrong. I found this on the web:
Transmission Line
Transformers. I am not sure how feasible it is to construct a transformer
like that.

Is there a good book that explains (pulse) transformer design in easy to
understand terms?

I would appreciate any suggestions you may have.

(From: Jerry Codner (gcodner@lightlink.com).)

Sounds like fun. What's it for?

(From: Chris Krah (chriskrah@apple.com).)

This is part of a power supply for a copper chloride laser. Conventional
power supplies use a mechanical chopper to generate the pulses. The
mechanical chopper looks similar to a rotary spark gap used in tesla coils
(neon sign transformer + rectifier + HV capacitor + chopper). I was looking
into more elegant solid state solutions. I have figured out most of the parts
for this power supply, except the transformer.

(From: Jerry Codner (gcodner@lightlink.com).)

Your problem with the ignition coil and with any transformer of this type will
be leakage inductance, but 50 us pulses at a 5 kHz rep rate isn't too bad.
How about flyback transformers? Essentially they are pulse transformers. You
picked a 20 ohm load impedance (200 volts/10 amps) so magnetizing inductance
should be 200 ohms or more at ~1/(2pi*50 us) = 3.2 kHz.

(From: Chris Krah (chriskrah@apple.com).)

Actually the input current will be much higher than that as the output peak
current is in the amp not mA range.

(From: Jerry Codner (gcodner@lightlink.com).)

How sharp do the edges have to be?

(From: Sam.)

Marco Lauschmann (laserlight@gmx.de) has pointed out that the 10 us figure for
pulse rise times quoted below is much longer than desired for the CuCl laser:

"The copper vapor laser needs sharp edged HV pulses, with rise-times in the
100 ns range! Power will be maximum at about 50 ns. It will still work
with a rise-time of 200 ns but the power will be much lower."

Therefore, the discussion below should be used only as a means of identifying
some of the relevant circuit design issues and possibly the basic math, not
for determining actual numbers or selecting components.

(From: Chris Krah (chriskrah@apple.com).)

10 us rise/fall time max.

(From: Jerry Codner (gcodner@lightlink.com).)

That will determine what the leakage inductance and secondary winding
capacitance must be. You will need to provide enough insulation for 20 kV
isolation (you should use 50% to 100% of the operating voltage in the
insulation thickness calculations) and the extra spacing increases leakage
inductance.

(From: Chris Krah (chriskrah@apple.com).)

I thought of immersing the transformer into high voltage oil such as Diala AX
from Shell.

(From: Jerry Codner (gcodner@lightlink.com).)

If we use 10 mH for the magnetizing inductance, obtained using a gapped core,
then a 200 volt input pulse will generate 1/2 amp of magnetizing current after
50 us. The gapped core is required to linearize the magnetizing inductance
and to provide for dc current since the input pulses are unipolar.

[Aside: You specified that the waveform as a capacitor discharge, if the RC
time constant is 50 us, then the final current is also 1/2 amp. That's
because the current is the integral of the voltage and the integral of an
exponential is equal to the initial value times the time constant, as though a
rectangular pulse were applied.]

For 10 us rise time, you need leakage reactance much less than R = 20 ohms or
L/R less than 10 us. For L/R = 1 us, L = 20 uH. That may be obtainable, but
you will have to keep sqrt(LC) below 1 us also and that will require C < 1 uF.
With a 100:1 turns ratio, C = 1 uf is within reason, but remember it's the
secondary capacitance reflected to the primary so that's 1uF/1002
= 100pF of secondary capacitance. A one layer secondary would be good, but
you are cranking out a bit of power (2 kW), so the conductors must be sized
for the peak current of 10 amps/100 = 0.1 amp and an rms current of
sqrt(50/200)*0.1 = 0.05 amperes. If the pairs of pulses are more widely
spaced than this, then resistance rather than rms current will detect the
conductor size.

(From: Chris Krah (chriskrah@apple.com).)

Isn't the gapped core also important to prevent the core from getting
saturated?

(From: Jerry Codner (gcodner@lightlink.com).)

Secondary capacitance is reduced by using tall, narrow windings, but this
increases leakage inductance. You will need to explore winding configurations
to find the optimal one for your situation. I would start with a
concentrically wound pair of windings, secondary over primary over core before
advancing to interleaved winding schemes that can lower leakage inductance but
then pose trickier voltage isolation problems. You need to keep the primary
turns low, so a high permeability core is essential, especially since it will
be gapped, thereby reducing the effective permeability.

If you can, the entire assembly should be immersed in dielectric fluid, e.g.
FC-77. Otherwise, you may have some severe insulation breakdown problems due
to corona.

(From: Chris Krah (chriskrah@apple.com).)

How about using e.g. 4 separate transformers, put the primaries and
secondaries in series. This way output capacitance and leakage inductance
could be minimized. Isolation would be much simpler this way.

I started hunting for more info on copper halide lasers and found
U.S. Patent #4,275,317: Pulse Switching for High Energy Lasers
by Laudenslager et al., June 23, 1981. It contains a really good description
of how a pulse forming network operates to produce multi-kV pulses with 10 ns
rise times, and also why such steep rise times are advantageous in driving
moderate to high pressure (e.g., excimer) laser systems.

This would appear to have significant relevance to driving a CuCl or CuBr
laser which requires two fast rise time pulses 150 us apart. It offers the
possibility of a totally solid state, SCR based power supply which is
relatively cheap and robust).

(From: Chris Krah (chriskrah@apple.com).)

You need 2 triggered spark gaps. The spark gaps operate at 10 to 15 psi air
pressure for better quenching.

You need 2 Neon Sign Transformers (NSTs) and 2 charge capacitors. Each charge
cap is directly connected to the output of the NTSs. No HV diodes are
necessary. That's the beauty of the system. The caps are charged via 60 Hz
cycle (similar to the setup in a tesla coil). The charge cap will form a low
pass filter with the impedance of the NST. The cutoff frequency of the low
pass filter is 60 Hz. Therefore the value of the cap can be calculated as
follows: fcutoff = 1/(2PI*Z*C) -> C = 1/(2PI*Z*fcutoff). Z is the impedance
of the NST and is equal to U/I where U is the open circuit output voltage of
the NST and I is the short circuit current of the secondary.

The caps are consecutively discharged into the cavity of the CuCl at both peaks
of the 60 Hz cycle. Therefore, the laser will operate at 120Hz.

The flyback transformer from a B/W TV or monochrome computer monitors can be
used to generate the trigger pulse and a monostable like the 74LS221 can be
used to provide the delay. These have low jitter and are continuously
adjustable. The wiring is simple with a minimum of external components. To
generate the trigger pulses, rectify low voltage AC via a bridge rectifier
and apply this to a peak detector (diode in series with cap, the cap will be
charged with the peak voltage minus forward voltage of diode). Pass the
rectified AC to the non-inverting input of a comparator, the peak voltage to
the inverting input. The comparator will generate your first trigger signal.
The latter also triggers the monoflop. The monoflop will generate second
trigger. The trigger signals are feed to the flyback trigger mechanism (SCR
based).

I know of a gentleman who has used this system successfully.
Operates quietly and can be tuned by adjusting the pulse spacing.

This certainly sounds like the way to go but the power supply may be very
non-trivial, not to mention, even more deadly than the mechanical pulser.

(From: Cody Martin.)

I was "fortunate" enough to have worked on the original Nerheim (forgive me
it's been 20 years) CuCl prototype. We acquired it by purchase from the
development work at CalTech. What is important is to note that if the
excitation pulse is well shaped (approximately square and 5 to 7 ns long)
so the dissociation pulse will also excite the laser transition.
This laser was put in the astable oscillator cavity form.
The resulting excitation is super-radiant (very very large gain
coefficient) and the small amount of light reflected back into the cavity
from the beginning of the spontaneous emission makes the need for a second
electrical pulse unnecessary. The device ran at 7 to 8.5 kHz and so produced
an rms power of 6 to 10 W. The peak power per pulse in a 1 to 2 cm
diameter beam was approximately 250 kW so extreme care must be taken
with the output.

We used High purity neon as a buffer gas. Adjustment of the base pressure
and CuCl density produced a 60 to 65% green/yellow laser for pumping a dye
laser. We ran the device for 4 years until the students doing their
experiment graduated. A simpler version was based on the same Blumlein
cable capacitor discharge system charged by the capacitor setup from
Anderson and Fitzsimmons for the Nitrogen laser.

(From: Steve Roberts.)

Once upon a time everybody was trying to make homemade double pulse copper
vapor lasers using CuCl or CuBr. Thats a major goof. I just got the
definitive Russian book on CuBr lasers on interlibrary loan. You have to go
the other way. Run them at several kHz in a quartz tube in a oven. Then you
don't need the double pulses as there is enough halide in the split state
that way - some of it recombining, some of it lasing. The power supply needs
about 14 kV at an amp or two. A close examination of a Spektronica CVL at
the Laser F/X conference confirmed it was running this way, at about 14 kHz.

Having just finished construction of a copper halide laser, I would hardly
call it an "easy" laser to build. Although this laser does not require the use
of expensive optics as some do, you must still have a vacuum system and
related apparatus, a source of helium, a way to accurately control the plasma
tube temperature at 400 C, a high voltage power supply capable of producing
two quick, accurately timed pulses at a high repetition rate and other items
which make this project somewhat more difficult than other types of lasers
featured in the Scientific American articles.

These are "Torch Lasers" (as opposed to the "Rocket Engine" chemical
lasers). :)

(From: Ken Donnell (kdonnell1@earthlink.net).)

If you want high output but are afraid of sophisticated optics and can
handle complex electronic design, try a copper vapor laser. I managed to
hack together a TEA copper vapor unit in only a few days by using an
acetylene torch to vaporize copper and blow it past a spark gap. This was
obviously only useful for short operating runs but produced over a watt of
510 nm and 578 nm. A high power version of the Blumlein circuit used in N2
lasers was all that was needed to trigger it.

(From: James Lockwood (james@foonly.com).)

I've been experimenting with both arc and oxy-acetylene heating of copper
(as an electrode with the arc and in a crucible for gas heating), attempting
to sidestep the high temperature issues within the discharge chamber. It's
pretty simple, just blowing monatomic Cu vapor and neon past a pair of long
electrodes with a mirror and OC at the ends. Exhaust goes into a chilled
vermiculite (flaked mica) substrate.

Obviously useless for long operating runs but still fun. It's not very
reliable, though, which is why I'm hoping for more detailed information on
how pressure and buffer gas mix affects operation. I've found quite a bit
of copper halide laser information online and in libraries, but very
little on TEA designs.

It uses pure copper vapor heated by a secondary arc (using carbon rods) and
carried past the primary electrodes by the injection of (Ne) buffer gas.
It's an extremely hacked-together design built over a couple of days purely
as a proof of concept. I'm not much of a laserist but I am experienced at
high voltage design and this was trivial to assemble.

The copper chloride laser will operate at the easier obtainable
temperature of 400 C. The gain on this type of laser is very high and
specialized optics are not necessary for successful operation. A simple
microscope slide will act as an output coupler. The operation of this
laser does depend on a pair of closely spaced high voltage discharges,
which may be the most demanding aspect of building this system.

(From: Steve Quest (Squest@cris.com).)

True, you need only one mirror in the Cu Vapor laser. True as
well about the high voltage pulses, which the differential time between
the two pulses HAS to be relatively exact, but this is not as hard as it
seems, as the timing can be done with cheap digital logic chips (decade
counters, and dividers) or with a programmable microcontroller to get the
timing exact. The *real* danger is the voltage AND the current are both
very high (read: lethal) so you MUST BE CAREFUL around them...........

(Ever seen a squirrel come across a 13.7 kV high wire power line? Well,
you'd be that squirrel. :)

Commercial copper vapor lasers do exist. Apparently, these lasers were
originally developed commercially for use in Uranium enrichment (!!) and
large scale adaptive optics (correcting for atmospheric distortions in large
telescopes). A high visibility application (no pun....) is in the laser
entertainment (light show) arena, well, at laest one - the high power Pink
Floyd laser show tours:

(From: L. Michael Roberts (NewsMail@laserfx.com).)

The source of all this wattage is a pair of Oxford Laser ACL 45 Copper vapour
lasers complete with a pair of British technicians who start up the lasers and
then leave for the hotel. These lasers have an output power of 45 watts in
pulsed mode with a repetition rate in the 5-10 kHz range and a beam diameter
in the 40 mm range. The beam diameter is intrinsically large as power output
scales with the volume of the plasma. While these specs would be a nightmare
for scanned graphics applications, they are perfect for beam effects.

While both CV and YAG lasers are pulsed making them unsuitable for graphics,
they have high power and high brightness and are ideal for beam displays. At
present, it is much simpler to set up and run a 50 watt YAG then a 50 watt CV
laser. Where the CV does win is the striking emerald green and gold colours
while the YAG is that harsh lime green.

Copper vapour lasers do not use a permanent gas fill as in argon and krypton
ion lasers. The lasing medium is a monatomic metal vapour obtained by heating
a plasma tube containing a metal charge using power from an electric
discharge. The discharge runs between electrodes at each end of a refractory
ceramic tube which is thermally insulated inside a vacuum envelope. The heat
generated by the repetitively pulsed discharge raises the temperature of the
tube sufficiently to vaporize the metal charge loaded along it's length. The
pulsed discharge then begins to excite the metal vapour in preference to the
flowing buffer (neon) gas and lasing action begins.

However with the flowing gas design, the wide 50 mm beam, their size and
weight, and their need to be fed copper rods at regular intervals, these are
not exactly ideal touring lasers.

There are now small, cheap, sealed tube design, air cooled CV lasers on the
market with up to 5 watts of power. One such unit from Spectronika sells for
about $7,000. The tubes last 700 to 800 hours and then are cheaply replaced.
(Well everything is relative - $1,650 for the replacement tube.) The
disadvantages are that they are pulsed and have a 14 mm beam diameter. None
the less, I expect to see many of these entering the entertainment market
shortly due to their reliability, colours, low cost, high power and ease of
operation. Compare that to argon ion laser prices!

Since this is a commercial product, I doubt that the manufacturer would be
giving away the schematics. However, Spectronika does sell the replacement
tubes for this laser for so you may want to consider buying a tube then
building your own power supply.

From: Patrick Dietzel (patrick@dietzel.net).)

One could calculate +9 spare tubes in from the start so your laser will
safely work for 7000 hours and costs $ 22k. Isn't too bad for 5 Watts sealed
air-cooled unused special, is it? It is also more reliable than a one-tube
solution. If the tube breaks or leaks you have a new one at hand immediately.
;-)

(From: Steve Roberts.)

That's right, the CVL economics are comparable if not better then the ion laser
economics.

I have seen the Spektronica unit and the thing is beautiful. A tube change out
would be a no brainer, unlike the 4 to 5 delicate hours of brain surgery it
takes to replace a delicate ion tube, during which time if you sneeze or even
sweat or have a tremor in your hand it could cost you dearly, either in terms
of output power or that $700 optic that just rolled off the bench or won't
clean up. The CVL tube has no fragile appendages, Brewster stems, and the only
cleaning is the OC face. There are no magnets, no leaks, no fragile ballast
feeds, and no major regulator circuitry. The CVL just has a on/off switch, no
current controls, etc. Thats one less failure, and the club staff can't burn
out the tube early by cranking up the power. (Hint: If you're selling a laser
to a club, open up the PSU and set the current limit to a little above the
optimum current for the tube)

For those who haven't done it, a large frame ion laser tube change is very
stressful, even if you do it every few days in a refurb shop. One wrong
move and your writing your customer a $5,000 to 17,000 check instead of
getting paid. One little water leak, and you find yourself doing about
$500 in parts and a few days labor redoing the PSU. Having factory service
do it for you is going to cost you $300 a hour, minimum $1000, plus travel
costs. That's why professional ion repairs are pricey, its not easy.

Another bonus is you get a new set of optics every change. 700 hours might
not be that long in a club situation, but figure in that at 1,500 hours in a
filthy club (I dare you to tell me there is a clean area backstage in
any club, except where they count the money) an ion tube is down to way
below initial power and getting quite dim. The external cavity optics, no
matter how well sealed, are shot. That's $1,400 right there. Lab grade Ion
doesn't mechanically hold up or stay stable when its got 6,000 watts of
subwoofer setting beside it either. That's why industrial lasers that are
sealed mirror or auto-tweek themselves are a blessing in a club.

Club owners are typically cheapskates, unless its a major corporation
that owns the club. Few of them understand planned maintenance. The club
owner might fork over enough for a refurb, but odds are he won't pay for a
new tube, and you'll find yourself out of a job if you tell him its 13,000$
to fix it. More likely he'll find another laserist who will buy him a cheap
used laser from some university and put it in, and he'll call you and say,
"Ha ha, I got the whole thing new for $5K." Unless it appears majorly
dim, he'll accept the used system on the basis of economics. Been there,
done that, got the T shirt. :)

Being told its $1,700 for a new drop in bulb is something am owner might
understand, especially when some of the HMI and Arc lamps in his
intelligent lighting fixtures cost around $400 to 500 each.

Also the gold and green from a small CVL have a very distinctive look, much
nicer then the lime green of 532 nm, and when the beam is still combined,
it looks a lovely golden white color, as when the eye is saturated with
yellow and blue-green it sees white.

The optimal pressure is 3 to 10 mbar for helium and 7 to 30 mbar for neon.
(1 mbar = 0.75 Torr.) It is important for a good discharge and also for
good overall efficiency that you use high purity gases.

Before I use the laser, I pump down the system overnight (at least 6 to 8
hours) with the built-in single stage rotary vacuum pump to less than
0.01 mbar. Then I control the system for leaks and fill in the buffer
gas (neon 4.5). I think, it is important to pump down the tube 2 to 3 decades
lower than the operating pressure to clean the tube of contaminants.
With a pump reaching only the operating pressure, I think you can not
clean the tube as thoroughly as required and the discharge may not be
stable.

When the laser was unused for a longer time (2 to 3 months or more), I
filled the tube 2 to 3 times with helium (because it is cheaper than neon)
to atmosphere pressure and pumped them down again also to clean the tube
of contaminants. A heated tube will be helpful to do this but is not a must.

The helium discharge should be a pink color. This indicates a clean discharge
(with neon it is orange-red). The discharge should be stable in the center
of the tube.

Output power at the green wavelength is nearly the same using helium or neon,
but neon produces a stronger yellow. The green:yellow ratio is dependent on
plasma tube temperature. Higher temperature results in less green and more
yellow. A ratio of 1.5 to 2 times green versus yellow is optimum (discharge
around 1,500 °C). To much yellow is also possible at lower temperatures,
when the discharge is not clean enough.

A short pulse width is also essential to achieve optimum output.
Measure the discharge pulse at the tube with a frequency compensated high
voltage probe (Tektronix P6015, 40 kV, 150 MHz). You need a pulse width
between 20 to 50 ns. When the discharge is to slow, the output power goes
down.

Unit is in excellent condition, but is slightly dusty from storage without
cases.

(From: Daniel Kapitan (dkapitan@jesus.ox.ac.uk).

Being one of the few copper laser junkies in the world, I was just wondering
what this one has done in the past. Also, it seems to me a bit unusual that
you can span the whole frequency range from 0 to 10 kHz. Surely your laser
will have died if the pulse rep gets below 1 kHz?

BTW, what the going rate these days?

(From: Brian Fluegel (bfluegel@nrel.gov).)

I agree that a copper vapor laser will die at rep rates below 1 kHz. It will
also frequently die at rep rates greater than or equal to 1 kHz. As a
graduate student, I suffered through four CVLs, and I could tell plenty of
horror stories. Are you a CVL junkie voluntarily?

"I need a UV laser source, but I'm wondering about using some existing laser
sources like Nd:YAG (1064nm), Ar +(488nm), Cu vapour (510nm). The trouble
is, I've NOT come across anyone who has doubled the Cu vapour laser in any
papers. Would that be a better option? Do you know of any crystals I could
use for the Cu vapour?"

Ramsey at the University of Macquarie in Australia has reported on
frequency-doubled copper vapor lasers (SPIE vol. 2062, pp22-29). There are
several references in this paper to other work with doubled CVL's.
Ramsey's group used BBO with the 510 nm line.

The copper vapor laser is an excellent pump for the a dye laser using R6G
as the lasing medium. The 511 nm line of the Cu laser is a relatively good
match to the absorption curve of R6G (its peak is at 530 nm). In a properly
designed system, pumping efficiency should be close to 40 percent.

In fact, the Cu laser is an excellent pump source for quite a few of the dyes
that lase from yellow to red. Rhodamine-620 is another one to try. :-) As the
name suggests, it lases at a peak of 620 nm in the orange.

Since the first high power pulses were observed at 510.6 nm from a copper vapor
laser almost 30 years ago there have been many different metals tried with
varying success. One of the more common types today is the helium-cadmium
(HeCd) laser. Some others that have been fairly efficient but have not found
widespread commercial value are: lead (Pb) vapor at 723 nm, barium (Ba) vapor
at 1,500 nm, manganese (Mn) vapor at 534/1,300 nm and gold (Au) vapor at 628
nm.

However, although the melting point of a metal like lead is relatively low,
the temperature at which adequate vapor pressure for laser action to take
place may be quite a bit higher. For instance, a copper vapor laser typically
must operate at 1,500 C but the melting temperature of copper is only 1,083 C.
Metal halide lasers have the advantage of working at much lower temperatures,
but because of the double discharge needed for their operation the power
supplies are quite complicated.

(From: ralord@webmail.co.za.)

The lead vapor laser works with a He buffer and produces very powerful and
short pulses of deep red with average powers of watts or tens of
watts, the gain is extremely high, it is no less than 40 dB single pass
for a 30 cm tube, and the laser works at 900-1000 °C with metallic lead.

A quartz tube is needed and so is a good high temp sealant and a way to
condense lead vapor before it reaches output windows.

With it's exceedingly high gain this laser has no need for resonators or
mirrors, a mirror would probably make it explode. :) They are superradiant
lasers, just like the good old nitrogen laser. It looks like LVLs are much
more efficient than CVLs and less demanding too.

(From: Bob.)

A lead vapor (i.e., atomic) laser can operate on at least 9 lines: 364 nm,
406 nm, 723 nm, 1,256 nm, 1,315 nm, 1,534 nm, 3,175 nm, 7,176 nm, and
7,942 nm. If you want to find the specific transitions, there is an energy
level diagram in "The handbook of Lasers" by Marvin J Webber.

(From: Harvey N. Rutt (hnr@ecs.soton.ac.uk).)

Actually *many* metals will lase in the vapour phase, gold certainly does, so
does lead, manganese, barium, strontium thallium, etc. I don't *recall* an
aluminum or calcium laser, but they very probably exist.

Basically whether something makes a good laser depends on the spectroscopic and
kinetic properties of its energy levels, which determine what wavelength the
laser will produce, how efficient it will be, whether CW or pulsed etc etc.
Also there are practical issues - the operating temperature of the gold vapour
laser is so high as to be a technical problem, for example. There was serious
interest in the Au vapour red line for medical applications (and the UV too)
but its was just too much of a pain, and simple red diodes came along....

The Cu vapour laser is popular simply because it produces useful (green and
yellow) wavelength outputs with decent efficiency (~1%) in a useful pulse
format, and they can be made reasonably well. Another significant factor is
that the output was useful for uranium laser isotope separation so money got
poured in to developing the Cu vapour laser (CVL) which helps!!

I suspect in fact that just about all the metals may well have lased in the
vapour phase; it's just that most are not very good, not much use, or there is
something simpler and cheaper! After all they all need to be pretty hot, a
disadvantage.

(From: Milan Karakas (mkarakas@vk.tel.hr).)

There is a zinc vapor laser with the strongest lines at 610.28 nm and
758.75 nm, and moderately strong lines at 747.83 nm and 761.18 nm. This
data is from the book by William T. Silfvast (sp?). I'm working on
constructing this laser but I need a better vacuum system. :(

(From: Leonard Migliore (lm@laserk.com).)

A copper vapor laser produces light from copper vapor. :-)

It's made by depositing copper metal on the ID of an alumina tube and heating
the tube to 1600 C inside a sealed cavity containing neon buffer (Obviously
you also need optics on both sides of the tube). Heating is generally done
with the same electrical discharge that pumps the copper. They run pulsed at
moderately high (4-20 kHz) repetition rates. Pulse duration is about 30
ns. The most fun thing about them is that they emit green (510.6 nm) and yellow
(578 nm) simultaneously, with about twice as much green as yellow. The least
fun about them is that the copper wants to plate out on the windows.

As you might expect, they start up slow since you have to run them for a long
time to get the copper boiling.

They use them at Livermore for isotope separation.

(From: Thomas A Suit (tsuit@mason2.gmu.edu).)

I remember an article in the Amateur Scientist column about a mercury vapor
laser with strong emission in the the green, and weak emission in the orange.
Since the vapor pressure of mercury is much lower than that of copper, it did
not have the high heat requirements. Yet I have not seen a commercially
produced mercury vapor laser. Why is that? What is the problem with it?

According to a book entitled: Metal Vapour Ion Lasers: Kinetic Processes
and Gas Discharges by I.G. Ivanov, E.L. Latush, M.F. Sem, and D.N. Astadjov
(Translator), ISBN: 0471955639,
when used in a pulsed positive column regime the strongest
HeZn lines lase on 491 nm and 492 nm. It runs at about
300 °C so a Pyrex tube can be used. A small tube about 25 inches
long with a 1/8" bore should be super radiant. You could probably use an
unstable resonator like some copper vapor lasers. Ordinary plane mirrors
and a microscope slide for an OC. You could scale the tube for a good
couple of watts of average power. The same book says that you can scale
a HeHg plasma tube to get a couple of watts average on the 567 line.
(which incidentally has a gain coefficiency of 100% per 4 inches in
the pure vapour). I was thinking you could use a similar SCR pulser
power supply to power the tube as is used with a CuBr laser. With
pulses the pumping level is instantaneously higher than CW but you
still have to pump the medium quite hard to reach
saturation, and hence you get more lines to lase that are far brighter than
in CW. Its not a metasatable laser so it won't create the same raw power as
copper. This book has a lot of information on Hg, Se, Cd, I, Zn, Ca, Sr, Sn.
etc. All the above mentioned elements apparently lase with strong lines in
the visible spectrum. I reckon there is enough sodium in table salt to lase.
It burns orange under a flame, and a flame is plasma after all. I saw a
sodium laser beacon which is a reddish/orange in colour. I don't know
how it is pumped though.

One thing I do know is that nearly all CW Metal vapor lasers are between 3
and 5 times as efficient as Ar lasers, although this may not translate into
power. In pulsed regime I think there is a lot of scope for a medium power
laser to be made. And as for Copper Bromide 1.5% to 2.5% efficiency. Just for
info a plasma tube 12" long 4.5 mm bore achieved 3.1 Watts average with
around 4.5 kW peak!!

See High Power Copper Vapor Laser in Operation
for one slightly larger than you could probably build, though not as powerful
as I thought when I first saw this picture. This was built at Lawrence
Livermore National Laboratory (LLNL) in the late 1970s.

(From: Bob.)

OK, I got bored and looked up some trivia about this laser:

Power output as an oscillator: 85 W.

Power output as an amplifier: 135 W.

Lifetime: About 300 hours.

Efficiency: On the order of 1.3% (as an amplifier).

Bore diameter: 73 mm.

Electrode spacing varied from 122 to 152 cm.

Operating temperature 1,450 °C (maintained solely by the discharge as
the bore is insulated).

I'm not sure where this came from nor which specific CuCl laser design it
applies to, but still could be useful.

(From: Unknown.)

I was doing some internet research
on the copper chloride laser and stumbled across your web site. I
have made an attempt at building one of these and failed. When I
was complete the thing was so big and noisy, that it wasn't very fun to
even try and operate. When I saw your plans for a solid state pulser
it really rekindled my interest. (See:
Solid State Pulser for CuCl or CuBr Laser.)
I really never documented it but I think that's
what I have. The capacitors C1 and C2 are quite critical. If these are too
big you won't get the required voltage swing of 15 kV AC which translates into
about 21 kV DC. I calculated these caps to be 5 nF each (total capacitance of
10 nF). Flybacks used in monitors usually have integrated rectifiers as
pointed out in the schematic. For the diodes D3 and D4 you may use microwave
oven diodes or build them yourself by putting 40 1N4007s in series to get the
appropriate PIV rating. C5 and C10 may have to be optimized for the flyback
used. Q1 and Q2 are 400 V, 20 A SCRs (which may be kind of OVERKILL). TR5 is
a step down transformer with 2 secondaries 9V/9V at 1 A or similar.
IC1A/B are monostables. Use the 74HC221 instead if possible.
Most of the components can be bought from
DigiKey.

Do you have an oscilloscope or similar? If not, I would really recommend
one for trouble shooting purposes. The basic concept is that the
caps C1 and C2 are charged during the positive half of the
AC cycle and discharged during the peak of the negative AC cycle.
A few modifications are necessary for this to work as intended as I had
drawn the schematics from memory and made a few mistakes. The trigger
input of the 74HC221 only sees the -1.2 V but it should see a positive pulse
for it to be triggered. Since the 74HC221 input is TTL compatible it needs
to see at least 2 V to get triggered. Remove C8 and connect output of U1A
directly to Pin 2 of IC1A. On an oscilloscope display, the voltage waveform
at pin 9 of TR5 on channel 1, and the output of U1A on channel 2.
You should see a nice 5 V trigger pulse on channel 2 during the positive
OR the negative half wave of the 60 Hz cycle (if that's the case, it's
a good sign.). Now put the output of IC1A (pin 4) on channel 1 of
your oscilloscope. Whenever there is a rising edge on the trigger
pulse (channel 2) you should see a negative going pulse on channel 1.
With R1 you should be able to adjust the width of that negative going pulse.
If that looks okay, remove R16 and substitute C8 with a 1K resistor.
The amplitude of the pulse at the output of U1A should now drop down to
about 3.9 V (may be more but you will notice a voltage drop). Now
insert a resistor in the line between IC1B and Q2. This is a current
limiting resistor and is required to prevent overloading the output of
IC1B. Please note that the secondary polarity of Tr5, Tr3 and Tr4
is quite critical.

Some questions and answers

Why is the flyback transformer preferred to the auto ignition coil?

A flyback tends to generate shorter output pulses. Car Ignition coils are
used in a low frequency application (less than 400 Hz) and tend to have a
high secondary layer capacitance.

How does the primary have to be modified in the flyback transformer?

Add about 10 turns of #18 AWG wire to the exposed leg of the transformer.
Use a transformer with an internal HV rectifier. These are commonly found
in monitors. The voltage output will be dependent on the number of primary
turns and type of transformer. As long as the transformer can break down the
trigatron spark gap (which is a piece of cake for the flyback
transformer) you are in good shape.

What is the value (output voltage, current) for the transformer?

T1 can be any transformer with at least 2 separate primaries, 9V/9V at 1 A,
I would suggest.

Does this circuit have variable frequency of just variable delay?
If not, could it be added?

Variable delay. Frequency adjustment could be added. The circuit used is
really simple since it takes advantage of the 60 Hz cycle.

Does the same transformer produce both closely spaced pulses?

No, two trigger transformers are needed.

Did you build a separate power supply for the +5 v?

No. You can generate the +5 V from the +9 V supply.

As you can probably tell I'm not an electrical engineer. Any other
tidbits you could offer on how the other gentleman got this to work so would
really appreciated. Also, you mentioned that "you do not guarantee the
successful operation of this circuit as it depends on many factors". What
are some of the factors that come in to play?

Mainly the trigatron, air pressure, gap width, etc. It takes some
experimentation but once you get it working you'll be pleased.

I am having trouble getting the SCRs to work. I hooked everything up
according to the schematics (less flybacks, the cap were wired directly to
ground) and hooked to scope up across the cap and it just shows a constant
charge. I checked the input pulse (from IC1, pin 5) with the scope and it
came out at +5 V for duration of 150 microseconds. I also tried pin 12 (-5 V)
with the same results. I believe it is hooked up correctly (K to ground,
G to the signal, and the other to the cap) but nothing happens.

Don't tie the right side of caps C5 or C10 to ground. The pulse current
through Q1 and Q2 may be too high. Use a 12 ohm series resistor or hook
it up to the flybacks. Secondly, make sure that that signal across R10 is
180 degrees out of phase with the positive half wave coming from pin 16 of
transformer TR5. If that's okay, proceed with Q1s and Q2s gate resistor.
Apparently the gate current (through the gate series resistors) is too small
to fire the SCR. For now I would suggest a 25 mA gate current, use a 180
ohm or smaller resistor. The voltage between gate and ground should
be 0.7 volts when a gate current is flowing. The voltage across the
gate resistor should therefore be (5V-0.7v)/25 mA = 172 (use 180 ohms).
You mentioned you used two monoflops to control the phase spacing.
Use the corresponding outputs on each monoflop (pin 5) to fire the SCRs
via 180 Ohm or smaller resistors. If this works, you are one step
closer to completion.

Do you think this could need sensitive gate SCRs?

A sensitive gate SCR would be better because it requires less gate drive
current, typically 500 uA. Non-sensitive gate SCRs require somewhere between
1 to 35 mA. However, with the SCR you have, drive current can be adjusted via
the gate resistor.

I got the pulse generation circuit working well. What modifications
would be needed to get this operating at 120 Hz?

I am glad you were able to get it to work. I am afraid that it would take
major modifications to get this circuit to work properly in 120 Hz mode,
although the circuit wouldn't necessarily become that much more complex.
I have not tried this yet, but I can take a shot at it. One modification
would be to run the comparator around U1A from both half waves rather a
single half wave. As you can see on the schematic I only use the positive
half wave (see Diode D7). In order to see both half waves you would need
another full wave rectifier (component B2). The monostable part would remain
the same. Q1 and Q2 would have to be trigger IGBTs. Those are
used in electronic ignitions. SCRs are not suitable for a "simple"
implementations since they remain on once they are triggered. The capacitors
C5 and C10 would have to be charged via resistor from DC voltage (off the 12 V
supply). Last but not least D3 and D4 would have to be substituted
by charge resistors. Even better would be separate NSTs, in that
case C1 an C2 could be larger and the charge resistors could be omitted.
The thing to consider here is that in 120 Hz mode, the spark gaps have
to be fired during the line AC zero crossing point. This is required
to cut power to the spark gap immediately after firing occurs. As
you know, the break down voltage of the spark gap drops once it is
triggered. If you would keep supplying it with power, the spark gap
would continue to conduct and that's what we want to prevent. This can be
compared to a HeNe Laser. Initial firing requires a high voltage pulse to
lower its breakdown voltage. The actual operating voltage is much
lower. To get back to the circuit, we are taking advantage of the
fact that there is a phase shift between the line AC voltage and the
voltage across those Capacitors C1 and C2. The magnitude of the phase
shift is dependent on the size of the charge resistors (here the impedance of
the NST) and the size of the charge caps C1 or C2. What this means
is that when the line AC reaches it's zero crossing point, the residual
voltage (which is also sinusoidal ) across C1 and C2 may be several KVs.
To my knowledge that's the effect we are taking advantage of. Hope
all above makes sense. It not, let me know.

Are trigger IGBTs expensive components and could they be purchased
from Digikey?

I haven't checked. I bought mine from Harris Semiconductor. The best match
would be a IRGS14B40L from
International Rectifier/.
This is an IGBT especially suited for ignition circuits. This IGBT
has an integrated voltage clamp and a low gate threshold voltage of 2 V.
Alternately, you may use a IRG4BC30F-ND available through Digikey.

I found the part number at Digikey (IRG4BC30F-ND) but had no luck with
the other number (IRGS14B40L) at irf.com.

Maybe they are no longer manufactured. I would hold off buying these now.
Get this circuit to work first and then you can make improvements later.

Lastly, if the AC voltage was at a small value when the caps fired
wouldn't the gap be shut off as the power dumped into the caps for
recharging?

That's correct, but because the charging characteristic of a capacitor is
exponential U(t) = U0 + exp(-t/RC). Initially, the cap may charge fairly
quickly, quick enough to sustain a spark gap discharge. That's of course
assuming that charging occurs in any other location than the zero cross over
point. In the zero cross over point the voltage is zero, no capacitor
charging can occur. Also you have to deal with circuit inductance.
Furthermore you want to establish a initial charging condition where the
voltage is zero.

Describe the capacitors you use for your laser?

I had built my pair of capacitors for the CuBr just prior to a rare find of
commercial high voltage capacitors for a VERY reasonable price. I will
substitute these commercial components (40,000 V, 15 nF) for the home-made
types when I do upgrade the system. For the original capacitors I used a
12" x 18" sheet of 0.015" thick copper-clad, Epoxy circuit board material.
One for each cap, with 3/4" of copper etched off from all sides. I rolled
these once lengthwise and inserted then into an 18" length of 4" diameter
PVC pipe to save space. These seemed to have worked quite well in this laser
system, but I was concerned about the dielectric breakdown at full power
from the 15,000 V NST. This never did happen, but it is certainly a very
real possibility. I employ an almost identical capacitor in my nitrogen
laser and it has been 'punctured' quite a few times at a potential of 15 kV
to 18 kV. You are correct in assuming that the thickness of the PCB material
will affect the self inductance (change of inductance change of dielectric
thickness^2) and hence the pulse width that this capacitor is capable of
delivering. In an N2 laser this parameter is critical for successful
operation. Although the CuBr laser does require a very fast, high energy
pulse. I doubt that it is as crucial in this system as in the N2 laser.
I would suggest that you try the stacked, thicker PCB capacitor construction.
The price of this material is not very great and it would be a very good
opportunity to test this theory.

You mentioned that CuBr is considerably harder to find than CuCl. Is it
worth my effort to try this route? How difficult would it be to add H2 to
the mix?

t's not so much a question of finding the copper (II) bromide but rather
finding a chemical distributor that will offer this chemical in a small
quantity. Most chemical companies only offer this in Kg sizes with $100.00+
price tags. Since the laser only requires a tiny amount of this chemical, I
could not justify the expense of a large quantity. I did finally find 25
gram quantities from Lancaster Synthesis, Inc. for $12.30. I am sure
that the HyBrID laser is quite feasible, although I have not tried adding H2
to the mix as yet. I have every intention of trying this experiment someday,
perhaps after the completion of a new pulse spacing unit. But as I say, I
have too many other laser projects going on right now to consider this.
I've been heavily involved with solid state laser construction projects of
late.

How do the physical and electrical values differ between the three
types.

This is something that would have to be determined by experimentation with
the three types. I can tell you that the parameters for operation of the
CuCl and the CuBr are very similar. I have no idea how the addition of H2
to the CuBr mix will affect the operating parameters, but supposedly it
boosts the efficiency by a nice margin.

Have you done any experimentation with triggered spark gaps? Can they
be made to operate reliable enough for this application?

I have not done any experimenting with any other type of power switching
device besides the pulser that I am using, but have considered the use of
triggered spark gaps. I believe this would be a viable option.

My discharge tube is a 24 inch length of 10 mm bore quartz glass with a 2 mm
wall thickness. The extra wall thickness was to insure good electrical
insulation between the HV discharge in the tube and the metal heat tape
wrapped around the length of the outside. I am operating the laser using
a single 15,000 V, 60 mA neon sign transformer with a rectified output. My
caps are home built from Epoxy circuit board material and have a capacitance
of approximately 12.5 nF each. The pulser's rotational speed is about 7,000
rpm, discharging these caps through the laser at approximately 115 Hz.

I have a good design of a triggered spark gap that I would be happy to
send you if interested. Also there is a good setup for situating those for
many heat, inductance, space, and time saving reasons. Thanks for the
help.

I would certainly be interested in any plans that you may have in regards
to a better PSU for this laser. I will get to this one of these days and
perhaps with a good design for an alternative power supply in hand, I would
become more motivated in this direction.

How does the CuBr laser compare to the CuCl laser as far as the
parameters are concerned?

Since I have only used CuBr in my laser I really cannot compare this to CuCl.
I have found that my laser seems to operate fine in the temperature range of
400°C despite the fact that CuBr does have a slightly higher melting
temperature. For the record, CuCl melts at 422°C and CuBr at 498°C.
Because these materials are under vacuum it appears as though vapors begin
to be released at a somewhat lower temperature than the melting points listed
above. A very obvious red/brown vapor (containing some free fluorine no
doubt) is noticeable being pumped through the vacuum lines at the temperature
climbs to the 400 C mark. This is certainly a good reminder that these
types of lasers MUST have their vacuum lines vented to the outside.

Do the triggered spark gaps need nitrogen, or hydrogen to be effective?
I plan to pressurize mine with air at about 15 psi.

No gas fill is necessary for the first test, but it improves the rise and
fall time of the pulses when you use nitrogen like I did. Hydrogen works
fine as well, but is dangerous due to its flammability. Even pure air
improves, because it consists mainly of nitrogen.

I lucked out today and Reynolds Industries agreed to send me some
'samples' of 5 nF, 30 kV mica caps. I think the guy took pity on me for my
capacitor misfortunes (not being able to find them for less than big $$$, or
a hefty minimum order). Do you think mica caps are low enough inductance
for this? I asked him and he said they aren't designed for low inductance
and he gave me a figure of 30 nH which, sounds very low to me. Maybe he meant
30 uH?

I don't think so. 30 uh is huge. 30 nH sounds more realistic. The capacitor
is series resonant at about 13 MHz (1/2*PI*sqrt(L*C)). May not be that
critical. In general, plate type caps are best. The inductance of rolled
type caps should scale linearly with the caps capacitance. Even if that
capacitor is a rolled type, the inductance may be low enough. After
all it's not a huge capacitor. I would suggest you give it a shot and if
it doesn't work, use Chris Chagaris' copper clad PCB idea.

I now plan to move on to the flyback transformers. You mentioned that
I would need to modify them by adding ten turns of wire to the exposed leg.
Is that connected to the primary, or is the original primary completely
disregarded in this setup?

The primary is completely disregarded. If you use the transformers primary
you will need to charge the caps up to 110 V.

How would I go about matching the capacitor values to the transformers?
Is there a formula that I would use to get close? Or do they really need to
be matched to operate efficiently?

This is actually the most difficult part as flyback transformers differ from
model to model. Yes, it is important to match the transformer coil
inductance to the size of the discharge caps. You want to make sure that
the discharge caps are completely discharged during the 60 Hz half cycle to
prevent the SCR from conducting during the capacitor C5 and C10 charging
period. SCRs once conducting, can only be discharged when the current into
the anode is smaller than a certain holding current, which is usually in the
lower amps (50 mA?). For this the caps have to be almost completely
discharged. You may have to do this by trial and error. I recommended
the use of flyback transformers because they are easier to get. In my
particular implementation I build the transformers myself to get sharpest
possible rise times. I used 2.2 uF, 600 V charging caps for C5 and C10.
The anodes of D1 and D2 came from the line AC directly (make sure the
polarity of the line AC is proper, better use a isolation transformer
and a series load, e.g. a halogen lamp in case shorting occurs. The SCRs
ground need to be connected to N (line AC ground return) NOT the phase in
case you don't use an isolation transformer. Be very careful! You
may have to swap pins 1 and 8 to get the correct line phase. The
transformer I built using an approximately 2 inch toroidal core (gray,
high permeability material). I first insulated the core, then put
100 turns of AWG 25 (or smaller) magnet wire around to create the
transformers secondary. Note - Make sure it is only one layer of magnet
wire to prevent HV breakdown. The primary is merely one turn of wire
(should be highly insulated). This way your output voltage is equivalent
to the number of secondary turns times the input voltage 100*150V(peak of
110 VRMS) which is about 15,000 V! I would strongly suggest that you use
this scheme as it yields more predictable output voltages and faster rise
times. The layer inductance in flybacks is quite high and reduces the
output rise times.

I like the idea of making my own transformer, as you did. I found a
few 2.5" OD x 1.5" ID toroids on-line with a permeability of 5,000
($12 each).

These are perfect for this application (perfect size + permeability).

How did you go about insulating your cores?

Usually Mylar tape is used. Another insulating scheme uses glass fiber cloth
wrapped around the core. After the primary and secondary are placed on the
core the latter is placed in a heated insulating varnish under a vacuum. The
glass fiber sucks up the varnish, creating a perfect insulation. Since I
didn't have Mylar tape nor glass fiber cloth I used conventional insulation
tape. Seemed to work fine. The ferrite has a pretty high resistance but
it's not infinite. If you use one layer of insulation tape and overlap
it properly that should be sufficient. If you really want to get carried
away you could immerse the core in HV oil to reduce corona but I don't
really see a compelling reason for doing that. All you need is a hefty
trigger pulse and that's what you'll get. Oh, and it looks great, too. I
should also remind that you need to connect a diode in series with the
secondary of the trigger transformer to prevent shorting of the spark gap.
I would recommend you use a BY713, 20 KV high voltage diode. I guess Philips
makes those. I bought mine in an electronics store on special order. On the
other hand, you may not need a diode if your trigger electrode is insulated.

What exactly triggers the gaps anyway? Does it need to be a full
discharge from the spark plug, or just the corona created as the caps
discharge through the pulse transformers? Either way, the air is ionized
thus triggering the main gap, right?

The high voltage pulse generated by your pulse transformer ionizes the medium
in your spark gap. This medium may be air or better, a noble gas, such as
helium (BTW: balloon helium is sold at K-Mart. Look for "The party machine").
Ionizing reduces the conductivity of that medium to a point where the main
spark gap (fed by the NST and HV charge caps) breaks down. Are you familiar
with photo flash units? The same principle applies. In photo flash units
the flash tubes glass body separates the auxiliary electrode and the spark
gap. You do not have to have contact with the medium to cause ionization.
So, even if you where to insulate your trigatron's trigger electrode, it
should still work.

Another thing I was thinking about was running the 120 V into a
voltage doubler, which would charge the caps. That way there could be
fewer turns on the secondary, or more on the primary.

Yes, but don't forget that you can only use one half wave for charging and
one half wave for discharging. Voltage doubling won't work because you rely
on both half waves. BUT you could use a 110 V to 220 V step up transformers.
Fry's sells small ones for 10 Bucks or so.

Have you tested yours?

Yes and they work great.

I don't know very much about magnetics. Is one turn on the primary
really sufficient to run this transformer?

Yes, if you use high permeability material. I know it sounds unbelievable
but it really works. Your inductance may be well above 1 uH. You may
calculate the inductance of your core by using the following formula:
L = Al * N2. Al is given in uH/1000 turns or something
similar. Also, don't forget that the discharge pulse is very short,
in the vicinity of microseconds. That's the beauty of this circuit, it's
simple, yet efficient.

Would the trigatrons be more reliable if an inductor was placed between
the discharge posts (at each end of the discharge tube) so the circuit would
operate similar to the N2 laser? In that setup the trigatrons would 'see'
the potential on the other side but once discharged it would fire through
the discharge tube?

I don't know how well that would work and I have never tried. I know what
you want to accomplish though. You want to pre bias the trigatrons similarly
to pre biasing a diode by putting a resistor at it's output. What is
commonly done to establish a biasing point is to put a high ohm resistor
in parallel to the spark gap to establish a small current flow through the
discharge tube (of course I forgot to add that to the schematic. I guess I
am getting old). This probably needs some experimentation. Distilled water
could be used for this purpose. My trigatrons are made from concrete, which
has a natural resistance of several 10s of Mohms. However, you
could apply the Blumlein (used in N2 lasers) circuit to the CuCl laser
as shown in the diagram that I attached to this email. In this
circuit, C1 = C2 = 2*Cm, because we want the series combination of C1 and
C2 to be Cm. If all caps are charged to, lets say 20 kV, the laser cavity
will see no voltage gradient, no discharge will take place. Now, if we
discharge C1 by triggering SG1, C1 will be discharged, which in turn will
discharge Cm to a level in which there is no voltage gradient across the
cavity. After C1 is discharge Cm's voltage level would therefore be 10 kV,
as C1 = C2. Now if C2 is discharged, Cm would get completely discharged.
I thought of this concept a year ago. Never simulated it. Don't know how
well this would work.